Urea resins based on urea-formaldehyde condensates in the form of foams to be used as thermal insulation, are known since quite some time and have been used for filling the hollow space of cavity walls and, for shaping into blocks and panels.
These foams are prepared by means of continuously or intermittently operating apparatuses consisting of:
a container (A) for the aqueous surfactant solution additioned with a hardner of the resin; PA1 a container (B) for the urea-formaldehyde resin (suitably dissolved or diluted and filtered); PA1 a pump, a compressor and a foaming-gun. PA1 (a) mixing together the formaldehyde with water and urea in such a ratio as to obtain a molar ratio formaldehyde-urea in the range of 2.2-1.7 at a pH=6.3-6.8; PA1 (b) then admixing hexamethylenetatramine ("hexamine") in quantities comprised between 2 and 15% by weight on the dry end product, at a temperature of 40.degree.-80.degree. C. and a pH equal to 8.5-10.0; PA1 (c) then heating up to 90.degree.-95.degree. C., adding an acid catalyst and by then condensing at 90.degree.-120.degree. C. at a pH of from 4.0 to 6.0, until attaining a viscosity of 50-120 cP (at 20.degree. C.); PA1 (d) neutralizing with sodium hydroxide up to a pH=6.0-7.0 and by then admixing urea up to a molar ratio formaldehyde-urea in the range of from 1.2-1.4; then condensing at 85.degree.-95.degree. C. until attaining a viscosity of 80-120 cP (measured at 20.degree. C.); PA1 (e) neutralizing with sodium hydroxide until reaching a pH of 7.5-9.0 and then cooling down to 25.degree.-30.degree. C.; PA1 (f) optionally drying the mixture in order to obtain a powdery product.
The apparatus works in such a way that the solution of container (A) will give place to a foam by blowing in air: this foam, which will act as a support or carrier, is mixed with the ureic resin solution, and the latter, in the presence of the hardner, will harden giving place to a stable solid foam.
In this way there will be obtained foams which in the humid state will have a density of 50-120 Kg/cu.mt. and in the dry state will have a density of 10-30 Kg/cu.mt. The full hardening and drying occurs in 20-30 days, depending on the temperature conditions, ventilation, and the permeability of the walls.
These foams have, however, the drawback of expelling considerable quantities of formaldehyde (1-6% by weight with respect to the dry foam) and thus cause discomforts and ecological problems both in the application step as well as during the drying.
This latter drawback is particularly felt when the foam is applied, as is often the case, to the hollow space or cavity in already inhabited buildings.
In the preparation of insulating foams based on ureaformaldehyde resins there may be used as starting material liquid resins (in solution) or solid resins which have to be dispersed in water shortly before use. The characteristics of the foams thus obtained have been recorded on Table 1.
The liquid resins offer the advantage of a lesser emission of formaldehyde by the foams obtained from them, but on the other side there are the drawbacks that they are less stable through time (see Table 2) and that they give foams which are less resistant to hydrolysis and have lower mechanical resistance.
The solid resins may be stored for a longer period before their use in the preparation of foams, and yield foams that are more stable through time and more resistant, but have a higher emission of formaldehyde.
The higher emission of formaldehyde by the foams obtained starting from solid resins is due to their higher molar ratio formaldehyde-urea (see Table 2). A lower molar ratio would reduce considerably the drawback of the expulsion of formaldehyde by the foam through time, but this cannot be achieved because said resins would not have the necessary stability in water.
This drawback was partly overcome by adding urea (15% to 25% by weight on the dry substance) or other formaldehyde-blocking agents to the aqueous resin dispersion shortly before use. The formaldehyde expelled during the drying phase may be thus reduced to 0.3%-0.9% by weight with respect to the dry foam.
The addition of urea involves, however, a considerable complication in the preparatory step, in as much as the urea has to be dissolved in the resin dispersion immediately before use; the dispersion itself must be heated in order to avoid precipitation due to the cooling caused by the urea dissolution and furthermore because there must be provided a control and correction of the pH which may have to be changed because of the addition of urea or other blocking agents.
Thus an object of this invention is to provide urea-formaldehyde resins suitably modified which, while preserving the advantages of the known resins (stability, low cost, simplicity of the processing equipment, high insulating power) yield foams possessing formaldehyde emission values much lower than those of the best foams known so far, and without involving the burdensome operations connected with the addition of urea immediately before the preparation of the foams.
By the process of this invention it is, moreover, possible to prepare ureic resins in the solid state, which once dispersed in water result in foams that together with the advantages already known for foams obtained starting from solid resins (good mechanical resistance, good stability through time), have the advantage of expelling very little formaldehyde during the drying phase.